Document WO 96/38538 teaches yeasts derived from improved S. cerevisiae strains which exhibit increased osmotolerance in that they give off gases (production of CO2) advantageous in loaf doughs containing high concentrations of sugar from 16% to 25% (baker's percentages). This document is silent as to the behavior of these yeasts in the presence of mold inhibitors, such as weak organic acids.
Document U.S. Pat. No. 4,318,991 discloses a process for the production of baker's yeasts which have a high fermentative activity in a baker's dough even in the presence of mold-inhibiting agents, such as weak organic acids, for instance acetic acid or propionic acid. This process comprises a step of adding such an acid, in particular during the multiplication of the yeast, in the final stage of its propagation so as to “adapt” the yeast to said acid, in particular present in the form of calcium propionate (hereinafter denoted Calpro or else cpp) in baker's doughs. That document does not specify the behavior of the yeasts thus produced in doughs with a high sugar content.
A document, in the name of the applicant, U.S. Pat. No. 4,346,115 describes a process for preparing a baker's yeast made resistant to weak organic acids, from an osmotolerant strain, by discontinuous addition of molasses during the final cycle of its multiplication.
Another document, also in the name of the applicant, EP 1559322, teaches strains for producing, after adaptation to weak acids, yeasts which are even more effective, according to tests well known to those skilled in the art, and a control strain which has been the reference for about twenty years for obtaining commercial yeasts that are effective on sweet doughs in the presence of mold inhibitors.
In addition to the fact that the adaptation, mentioned above, to weak acids, in the form of calcium propionate for example, during the multiplication of the yeast, has certain costs, the latter can also have a negative impact on the fermentative activity of the yeast, in particular in the presence of sweet or highly sweet doughs.
The unfavorable common point of these prior art techniques aimed at improving the resistance of baker's yeasts to mold-inhibiting weak acids in sweet or non-sweet baker's doughs, by adapting the strains from which they are derived to said weak acids, is that they confer on the yeasts only a provisional/transient and non-permanent resistance to weak acids. The adaptation consists in exposing the yeast cells to a sublethal amount of a weak acid during their growth phase; this prior exposure subsequently enables the adapted cells to tolerate the presence of the same weak acid better when they are placed under fermentation conditions. In certain cases, a given weak acid can confer an adaptation with respect to the subsequent presence of another weak acid in the fermentation phase. These notions are exemplified in the article by Ferreira et al. (1997) International Journal of Food Microbiology 36, 145-153. The mechanisms of the adaptation to weak acids have been the subject of several studies. Some of these studies have been collated in the review article by Piper et al. (2001) Microbiology 147, 2635-2642. It is learnt therein that the adaptation is based on cell mechanisms involving several proteins, including certain membrane pumps. These mechanisms are non-genetic and essentially transient: they are set up by the cells as a reaction to the stress represented by the weak acid, but end up disappearing in the absence of this stress: the cells previously adapted return to the nonadapted cell state.
Other, much more complex, processes, such as those described in documents EP 645094 or WO 99/51746, teach a genetic modification of strains capable of producing baker's yeasts so as to make them intrinsically resistant to weak acids.
The applicant, continuing its studies to search for strains which are more effective still, has noted that a particular strain derived from its internal collection of osmotolerant strains has a genetic inheritance such that, if it is hybridized or mutated, according to conventional processes moreover, this provides a strain capable of producing a baker's yeast which is particularly effective and which, entirely surprisingly, does not require any adaptation before it is introduced into highly sweet doughs containing a weak acid in the form of calcium propionate (denoted Calpro, or cpp), for example.
It is this discovery which forms the basis of the present invention.